Having an app would actually be an interesting concept. I'd still keep the same kind of integrity of the Dream World more than likely, but without the limitations of being forced to sit at a computer to do it. As far as what else the app could do, maybe some potential things would be the ability to have a Pokéwalker again, maybe something more for data purposes, or if you find something out there that you could potentially scan like a QR code that could unlock something small for you, like items or a Pokémon (nothing crazy though)?